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United States Patent |
6,107,909
|
Kosinski
|
August 22, 2000
|
Trimmed surge resistors
Abstract
An electrical resistor having a resistance value and capable of
withstanding high power surges, utilizing a thick film deposited on a
substrate and trimmed with one or more cuts configured to maintain a level
of current crowding while increasing the resistance value of the resistor.
A surge resistor can be modified in a similar fashion.
Inventors:
|
Kosinski; John P. (Menomonee Falls, WI)
|
Assignee:
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Microlectronic Modules Corporation (New Berlin, WI)
|
Appl. No.:
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255484 |
Filed:
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February 23, 1999 |
Current U.S. Class: |
338/195; 29/610.1; 338/20; 338/22R |
Intern'l Class: |
H01C 010/48 |
Field of Search: |
338/195,20,21,22 R
29/610.1
|
References Cited
U.S. Patent Documents
3846731 | Nov., 1974 | Dumas et al.
| |
4146673 | Mar., 1979 | Headley | 428/335.
|
4475099 | Oct., 1984 | Praria.
| |
4859981 | Aug., 1989 | Peschl.
| |
4929923 | May., 1990 | Dharmadhikari.
| |
4999731 | Mar., 1991 | Bender et al.
| |
5043694 | Aug., 1991 | Higashi et al. | 338/195.
|
5184108 | Feb., 1993 | Bloom et al.
| |
5198794 | Mar., 1993 | Sato et al.
| |
5363084 | Nov., 1994 | Swinehart.
| |
5428339 | Jun., 1995 | Das.
| |
5436609 | Jul., 1995 | Chan et al.
| |
5493148 | Feb., 1996 | Ohata et al.
| |
5504470 | Apr., 1996 | Ginn.
| |
5874887 | Feb., 1999 | Kosinski | 338/195.
|
Other References
Hybrid Circuit Technology "Laser Technology, Dispensing Equipment " Jul.
1987, authors Alan and Cable Ann Rogers, Electro Scientific Industries,
Inc., Portland Ore.
|
Primary Examiner: Gellner; Michael L.
Assistant Examiner: Lee; Richard K.
Attorney, Agent or Firm: Reinhart, Boerner, Van Deuren, Norris & Rieselbach, s.c.
Parent Case Text
This application is a continuation in part of Ser. No. 08/917,972, filed
Aug. 27, 1997 now a U.S. Pat. No. 5,874,887.
Claims
I claim:
1. A method of modifying a surge resistor, said method comprising:
providing a thick film high voltage surge resistor with an initial
resistance value, said thick film trimmed with a U-cut having endpoints
outside said thick film to provide a trimmed film portion on one side of
said U-cut and a resistive film portion on the other side of said U-cut;
and
coupling a J-cut with said U-cut on said resistive film portion, said J-cut
having a leg portion along and a distance apart from the leg portion of
said U-cut, an endpoint outside said thick film, and a curved connector
coupled to a curved corner having a radial dimension like the curved
portion of said U-cut, said J-cut increasing the resistance value of the
resistor.
2. The method of claim 1 wherein said J-cut leg portion has a variable
length dimension.
3. The method of claim 1 further including an additional J-cut joined to at
least one of said U-cut and coupled J-cut.
4. The method of claim 1 wherein a generally linear section couples said
J-cut to said curved connector.
5. The method of claim 4 wherein said J-cut has a variable length
dimension.
6. A method of maintaining a level of current crowding in a trimmed surge
resistor while increasing the resistance value of the resistor, said
method comprising:
providing a high voltage surge resistor with an initial resistance value,
said resistor rimmed with a n initial cut having endpoints outside said
resistor to provide a trimmed portion on one side of said cut and a
resistive portion on the other side of said cut;
cutting a groove within said resistive portion and along said initial cut,
said groove having an initial endpoint outside said resistive portion; and
extending said groove to said initial cut with a curved cut extending into
the current pathway of the resistor and having a predetermined radial
dimension to maintain the level of current crowding.
7. The method of claim 6 wherein said groove has a variable length
dimension.
8. The method of claim 6 further including cutting an additional groove and
extending said additional groove to said initial cut to further increase
the resistance value.
9. The method of claim 6 wherein said initial cut includes at least one
plunge cut from the edge of said trimmed portion to said curved portion of
said initial cut.
10. The method of claim 6 wherein said initial cut is a U-cut.
11. The method of claim 10 wherein said cut includes a comb cut therewith.
12. A resistor capable of withstanding high power surges, comprising:
a resistive thick film on a substrate between a pair of electrodes;
a U-shaped groove in said thick film dividing said thick film into a
resistive portion and a trimmed portion, said U-shaped groove having
endpoints outside said thick film and a radius of curvature to control
current crowding; and
at least one J-shaped groove in said resistive portion and coupled to said
U-shaped groove, said J-shaped groove having an endpoint outside said
resistive portion, a curved corner, and a curved connector portion, which
follow the contour of the U-shaped groove wherein said J-shaped groove
increases the resistance value of said resistor.
13. The resistor of claim 12 wherein said U-shaped groove includes at least
one plunge cut from the edge of said trimmed portion to said curvature of
said U-shaped groove.
14. The resistor of claim 12 further comprising a generally linear section
coupling the curved corner to the curved connector portion.
15. The resistor of claim 12 having a plurality of J-shaped grooves,
wherein at least one said groove is coupled to said U-shaped groove.
16. The resistor of claim 15 having two J-shaped grooves, wherein each said
groove is coupled to said U-shaped groove.
17. The resistor of claim 12 wherein said curved corner and said curved
connector portion each has a radius of curvature like the radius of
curvature of said U-shaped groove.
18. An electrical resistor component having a precise resistance value and
capable of withstanding high power surges, said resistor component of the
type in which a thick film is deposited on a substrate and trimmed with a
U-cut dividing the thick film into a resistive portion and a trimmed
portion, the improvement comprising:
at least one J-cut providing a trim to a desired value, said J-cut having
an endpoint outside said resistive portion, a leg portion and a curved
portion extending along and apart from the leg portion of said U-cut into
the current pathway, and a curved connector portion coupling the J-cut to
the U-cut.
19. The resistor component of claim 18 wherein said J-cut leg portion has a
variable length dimension.
20. The resistor component of claim 18 wherein said J-cut includes a linear
section with a variable length dimension.
Description
BACKGROUND OF THE INVENTION.
This invention relates generally to trimmed surge resistors for electrical
circuits and, more specifically, to the modification of such resistors to
achieve precision tolerances.
The development of various ceramic and/or cermet materials made possible
many of the recent developments in resistor technology. Such materials are
very stable at high temperatures and capable of withstanding temperature
and voltage extremes. It is possible to customize these materials to
provide resistance values which range from an ohm or less to megaohm
values. Such variations in function may be incorporated into the materials
during manufacture and/or during the associated firing and/or curing
processes. A particular benefit of these materials is that they can be
applied to a variety of circuit substrates using well-known thick film
techniques such as screen printing.
Neither the materials nor their application lend themselves to an exacting
resistance value. As a practical matter, excess material is applied to,
for instance, a substrate, then trimmed or removed from the resistor.
Removal techniques have become more sophisticated in recent years and now
encompass the latest laser technology. Any trim or cut must be made such
that the resulting resistance value is less than or equal to the value
desired, as additional trimming can only increase the final value.
Regardless of the technique, the material removed must provide the desired
resistance value.
Various concepts related to the trimming of excess material are provided by
way of the prior art, in particular in U.S. Pat. Nos. 5,043,694 and
5,504,470--both of which are incorporated herein by reference in their
entirety. The importance of resistor trimming is amplified in the
situation where there can occur large electrical surges across a circuit.
Survival of the resistor requires a high quality material and thick film.
Compositional deficiencies or defects in the application may lead to
failure under high surge conditions.
U.S. Pat. No. 4,528,546 discusses resistors in the context of high surge
conditions. Incorporated herein by reference in its entirety, this prior
art patent illustrates, schematically, a typical resistor component.
Electrical terminals can be formed by screen printing a conductive
composition on a ceramic substrate. A ceramic and/or cermet material can
then be screen printed on the substrate to connect the terminals.
Typically, the initial resistance value of a resistor will be known,
calculated based on the number of ohms per square of resistor, where a
square is one unit of equal length and width. Decreasing the effective
width of a resistor to one-half the original will double the number of
squares and simultaneously double the resulting resistance value. Trimming
can be conducted with intermittent resistance measurements. However, for
precise resistors, resistance is monitored while the trimming proceeds.
For instance, when a desired resistance level is reached, the laser
trimming device can be disengaged to prevent further removal of resistor
material.
U.S. Pat. No. 4,528,546, as referenced above, illustrates a simple plunge
cut into the resistor. This method of trimming is quite simple and serves
sufficiently to increase resistance value. However, a large voltage
gradient, resulting from the redirection of current, will invariably exist
across the cut. In addition, it has been shown that the current crowds
into a region surrounding the terminus of the cut opposite the end point
on the resistor edge. In addition, during a surge condition, the voltage
gradient may cause arcing across the cut/trim, arcing of the sort which
could also destroy the resistor.
Plunge cuts can be used alone or in combination with a variety of scan
cuts. Scan cuts are made parallel to the current flow and alleviate
current crowding conditions. The cut/trim is relatively simple and the
increase in resistance directly calculated. When used in combination with
one or more plunge cuts, current is prevented from flowing into the
resulting trimmed portions.
U.S. Pat. No. 5,504,470 describes combining an initial terminal to terminal
scan cut with a plurality of plunge cuts, otherwise referred to as a comb
cut. The comb cut is described as preventing arcing due to excessive
voltage gradients across each plunge. The cuts and/or trimming is
described in conjunction with many different prior art resistor devices
and a variety of removal methods, all of which are specifically
incorporated by reference herein in their entirety. The cuts are straight
and easy to incorporate. Because there are no endpoints in the resistive
portion of the film.
U.S. Pat. No. 5,043,694, as referenced above, describes a variety of
cuts/trims of the prior art, including various u-shaped, square and
semi-circular cuts. This patent departs from the prior art in describing a
trimming groove having two curved portions linked by a straight portion
parallel to a linear resistor edge. The resulting configuration resembles
the outline of a paper clip placed along the edge of a resistor, with the
curved portions directed toward the electrodes. Various cut/trim
configurations are also described inside the outline to further precisely
trim or adjust resistance values.
Nonetheless, the prior art has associated with it various problems and
deficiencies. With reference to the resistor described in the '470 patent,
exacting precision is required in the trimming to eliminate any film on
either end of the scan cut or between the plunge and scan cuts, to prevent
extreme current crowding and certain circuit failure. This situation is
avoided with the resistor described in the '694 patent: curved cuts avoid
the residual film problem. However, trimming in this manner is relatively
time-consuming. Regardless, with either a scan cut or a curved cut,
resistance is, in large part, controlled by the initial cut. Further
trimming to increase resistance precisely to a desired value is difficult.
There is a need for a trimmed surge resistor and method for its
preparation, to provide benefits not otherwise possible.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show, for purposes of comparison, electrical components of
the prior art trimmed with a U-cut; in particular, FIG. 1B shows
schematically the current path induced by such a U-cut of the prior art.
FIGS. 2A-2C are schematic representations of electrical resistors and/or
resistor components, in accordance with this invention and as can be
prepared therewith: FIG. 2A shows a U-cut with a coupled J-cut and/or
J-shaped groove; FIG. 2B shows a U-cut with a J-cut/J-shaped groove
coupled to each leg thereof; and FIG. 2C shows a U-cut with a plurality of
J-cuts/J-shaped grooves coupled in succession to one leg thereof. FIG. 2A
shows schematically that the current pathway of FIG. 1B is not
incrementally or otherwise crowded by additional trimming, also in
accordance with this invention.
FIGS. 3A-3C are also schematic representations of electrical resistors
and/or resistor components similar to those representations of FIGS. 1A-1C
but including one (3A) or more (3B and 3C) plunge-type cuts, also in
accordance with this invention and as can be prepared therewith.
FIG. 4 illustrates another initial cut of one type which can be utilized
with the present invention.
FIGS. 5 and 6 are a schematic representation or electrical resistors
illustrating a U-cut coupled with an extended J-cut in accordance with the
present invention wherein FIG. 5 further includes a series of plunge-type
cuts and FIG. 6 includes an extended J-cut in combination with a J-cut.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a resistor
component for protection against over voltage and/or surge conditions and
methods for the preparation of such a component, overcoming various
deficiencies and shortcomings of the prior art, including those outlined
above. It will be understood skilled in the art that one or more aspects
of this invention can meet certain objectives, while one or more other
aspects meet certain other objectives. Each objective may not apply
equally, in all instances, to every aspect of the invention. As such, the
following objects--in light of the prior art regarding trimmed
resistors--can be viewed in the alternative with respect to any one aspect
of the present invention.
It is an object of the present invention to provide a method of trimming
precisely a surge resistor with tight tolerance limits, and/or without
further trimming an initial scan or perimeter cut.
It can also be an object of the present invention to provide a quick,
efficient method for trimming a resistor component which permits trimming
to a desired resistance value without further trimming of an initial scan
or perimeter cut.
It can also be an object of the present invention to provide a trimmed
resistor component and/or a method for further trimming which does not
increase current crowding, which does not place an endpoint of a cut
within the current path, which does not cause micro-cracking of a
resistive material, and/or which does not promote a change of resistance
value under surge conditions.
It can also be an object of the present invention to provide a trimmed
resistor and/or method for further trimming which prevents increased
current crowding and arcing between trimmed portions of a resistive
material.
It can also be an object of the present invention to provide a method of
trimming a resistive material to achieve a desired resistance value which
permits resistance to be monitored during the trimming maneuver and the
cut trimmed without an endpoint within the current path.
The resistors, components and/or methods of the present invention can
suitably comprise, consist of, or consist essentially of various elements
and/or components, including those specifically described herein. Each
such resistor, component and/or method is distinguishable,
characteristically contrast, and can be practiced in conjunction with the
present invention separate and apart from another. Accordingly, it should
be understood that the inventive compositions and/or methods, as
illustratively discussed herein, can be prepared and/or practiced in the
absence of any one element, species and/or step which may or may not be
specifically disclosed, referenced or inferred herein, the absence of
which may or may not be specifically disclosed, referenced or inferred
herein.
Other objects, features and advantages of the present invention will be
apparent from this summary and the figures and examples thereof,
especially so to those skilled in the art having knowledge of the
preparation of such resistors and related components. Such objectives,
features, benefits and advantages will be apparent from the above as taken
in conjunction with the accompanying figures, examples, data and all
reasonable inferences to be drawn therefrom.
In part, the present invention is a method of modifying a surge resistor.
The method includes (1) providing a thick film high voltage surge resistor
with an initial resistance value, the thick film trimmed with a U-cut
having endpoints outside the thick film to provide a trimmed film portion
on one side of the U-cut and a resistive film portion on the other side of
the U-cut; and (2) coupling a J-cut with the U-cut on the resistive film
portion, the J-cut having a leg portion along and a distance apart from
the leg portion of the U-cut and an endpoint outside the thick film, and a
curved portion having a radial dimension like the curved connector portion
of the U-cut. Without limitation, such a method can provide a
predetermined resistance, with the current crowding remaining
substantially unchanged. The leg portion of the J-cut can have a length
dimension which varies depending upon the initial resistance value and the
modification desired. In preferred embodiments, the leg portion of the
J-cut can be substantially parallel to the leg portion of the leg U-cut
and/or the curved portion of the J-cut can have radial dimension
substantially the same as that of the curved portion of the U-cut. Current
crowding remains substantially unchanged where, when using the present
invention, the combination of the J-cut leg and curved portions do not
extend beyond the connector portion of the U-cut.
Alternatively, in applications where a greater degree of trimming is
required to reach the predetermined resistance value, the J-cut leg can be
extended beyond the connector portion of the U-cut. In these embodiments,
a second curved portion in the J-cut couples the J-cut to the U-cut. Both
the first and second curved portions of the J-cut preferably have a radial
dimension substantially the same as that of the curved portion of the
U-cut to minimize current crowding in the resistor component. Preferably,
the first and second curved portions are joined by a generally linear
portion which is substantially parallel to the linear base section of the
U-cut. To further minimize current crowding in the resistor, the second
curved portion preferably extends only a relatively short distance past
the linear base section of the U-cut.
Preferred embodiments of the present invention can also include coupling a
plurality of J-cuts. After an initial J-cut coupled with a U-cut, another
J-cut can be joined to one or both of the coupled J-cut and U-cut. Whether
or not an additional J-cut is incorporated, the J-cut can include at least
one plunge-cut from the edge of the trimmed portion to the curved portion
of the U-cut. In highly preferred embodiments, a plurality of such
plunge-cuts are incorporated such that the trimmed portion inside the
U-cut includes a comb cut.
In part, the present invention is a method of maintaining a level of
current crowding in a trimmed surge resistor, while increasing the
resistance value of the resistor. The method includes (1) providing a high
voltage surge resistor with an initial resistance value, the resistor
trimmed with an initial cut having endpoints outside the resistor to
provide a trimmed portion on one side of the initial cut and a resistive
portion on the other side of the cut; (2) cutting a groove within the
resistive portion and along the cut, the groove having an endpoint outside
of the resistor; and (3) extending the groove to the initial cut with a
curved cut having a radial dimension. The curvature does not extend the
groove into the current pathway of the resistor. Preferred embodiments
include those mentioned above, particularly with respect to the initial
cut, the groove, the curved cut, and the preferred relationship thereof.
The initial cut is preferentially curved, having endpoints outside the
resistor. In highly preferred embodiments, the curved initial cut is a
U-cut and the groove is the leg portion of a J-cut, which is extended to
the U-cut with a curved cut having a radial dimension like that of the
curved portion of the U-cut.
In other preferred embodiments, the groove can be cut with a length
dimension to meet the modification desired of the initial resistance
value. Likewise, in preferred embodiments, a U-cut can include at least
one plunge cut from the edge of the trimmed portion to the curved portion
of the U-cut. In highly preferred embodiments, there are a plurality of
plunge cuts such that there is formed a comb cut in the trimmed film
portion defined by the U-cut. Regardless of the presence of one or more
plunge cuts, a highly preferred embodiment can include additional parallel
groove cutting and extension to the U-cut. Alternatively, such cutting and
extension can be made with respect to a previously cut parallel groove.
In part, the present invention is a resistor capable of withstanding high
power surges. The resistor includes (1) a resistive thick film on a
substrate between a pair of electrodes; (2) a U-shaped groove in the thick
film dividing the thick film into a resistive portion and a trimmed
portion, with a U-shaped groove having endpoints outside the thick film
and a radius of curvature to control current crowding; and (3) at least
one J-shaped groove coupled to the U-shaped groove, with the J-shaped
groove having an endpoint outside the resistive portion and a radius of
curvature like the radius of curvature of the U-shaped groove. Preferred
embodiments can include those features described above with respect to the
position of the leg portion of such a J-shaped groove and its radius of
curvature, as compared to the U-shaped groove.
In preferred embodiments, the U-shaped groove includes at least one plunge
cut from the edge of the trimmed portion to the curvature of the U-shaped
groove. In highly preferred embodiments, a plurality of such plunge cuts
provides a comb cut in conjunction with the U-shaped cut. Likewise,
preferred embodiments can also include a plurality of J-shaped grooves,
such that at least one of the grooves is coupled to the U-shaped groove.
Where at least two U-shaped grooves are present, it is highly preferred
that one such groove is coupled to the U-shaped groove.
In part, the present invention is an electrical resistor component having a
precise resistance value and capable of withstanding high power surges.
The resistor component is of the type in which a thick film is deposited
on a substrate and trimmed with a U-cut which divides the thick film into
a resistive portion and a trimmed portion; such a resistor component can
have an improvement including at least one J-cut which provides a
post-trim tolerance of about .+-.0.05%, with the J-cut coupled to the
U-cut and having a leg portion substantially parallel to the leg portion
of the U-cut, an endpoint outside the resistive portion, and a curved
portion having a radial dimension substantially the same as the curved
portion of the U-cut.
In preferred embodiments, the leg portion of the U-cut has a variable
length dimension, for precise modification of initial resistance value.
Alternatively, the resistor component can include a plurality of J-cuts,
at least one of which is coupled to a U-cut. Whether or not there are a
plurality of J-cuts, the U-cut can include either a plurality of plunge
cuts or a comb cut within the trimmed portion defined by the U-cut.
With reference to the figures, the methods and apparatus of this invention
can be understood in the context of a resistor component. As shown in FIG.
1A and those which follow thereafter, component 10 has electrical
terminals 12 and 14 with resistor 16 therebetween. It is understood that
terminals 12 and 14, together with resistor 16, will typically be formed
as thick film materials upon a substrate, although there is no specific
requirement thereof Even so and without limitation, material choices for
the substrate, terminals, and resistor will be well known to those skilled
in the art. Various embodiments can include any alumina substrate with
palladium-silver electrodes and a cermet resistive material screen-printed
thereon.
Preferred embodiments of the present invention can be utilized in
conjunction with various trims and cuts of the prior art. For purposes of
illustration and with reference to FIG. 1A, resistor 16 can be trimmed
with a U-cut having leg portions 18 joined by linear cut 20. The U-cut
increases the resistance value of resistor 16 and divides it into trimmed
and resistive portions 22 and 24, respectively. Current pathways 26
between terminals 12 and 14 are schematically represented in FIG. 1B and
are directed around the periphery of the U-cut, with crowding in the area
of the resistive portion in the proximity of the curved portions of the
U-cut. As mentioned above, such regions will heat very unevenly and may
cause the resistor to destructively fail. As previously mentioned, it is
an object of the present invention to provide a method to further increase
resistance value without contribution to current crowding. It will be
understood by those skilled in the art that the trims/cuts illustrated in
the figures are shown by lines which represent the three-dimensional
removal of material from the resistor.
Referring to FIG. 2A, resistor 24 is trimmed with a U-cut having leg
portions 18 joined by linear cut 20 at curved connector portions 28.
Additional resistance can be imparted by modifying resistor 24 with
cut/groove 30 at a distance apart from leg portion 18. In preferred
embodiments, cut/groove 30 is substantially parallel to leg portion 18.
Regardless, cut/groove 30 is coupled to leg portion 18 by curved
cut/groove 32. Cut/groove 30 can have a variable length dimension
depending upon the incremental resistance to be imparted. With reference
to current pathways 26, it will be understood, however, that the
combination of groove/cut 30 and curved portion 32 does not extend beyond
curved connector 28 of the U-cut and/or into the current pathway.
Another embodiment of the present invention is illustrated in FIG. 2B. A
plurality of trims are coupled to the U-cut, with such a configuration
useful to incrementally increase the resistance value of the resistor
beyond that initially imparted by the initial cut. As shown in FIG. 2B,
cuts/grooves 30a and 30b are made a distance apart from the respective leg
portions 18 of the U-cut. Again, in preferred embodiments but not
necessarily so, cuts/grooves 30a and 30b are substantially parallel to
such leg portions. Regardless, they are coupled and/or joined to leg
portions 18 of the U-cut and, in order to prevent further current
crowding, do not extend beyond curved connector portions 28.
FIG. 2C illustrates another embodiment of this invention. A series of
J-shaped cuts/grooves can be used in conjunction with leg portion 18 of an
initial cut. In such a way, resistance can be increased by making
cut/groove 30 in resistive portion 24 and extending it until such a time
when the final resistance value is met by extension thereof with curved
portion 32 and coupling with leg portion 18. If an incremental amount of
resistance is required another cut/groove 34 can be made and extended to
approach the desired resistance value, at which point curved portion 36 is
used to couple or join cut/groove 34 with cut/groove 30. The radial
dimension or degree of curvature of curved portion 36, curved portion 32
and curved connector 28 are sufficiently alike so as to not extend
cuts/grooves 30 and 34 into the current pathway imposed by the initial
cut. In preferred embodiments, such radii or degrees of curvature are
substantially the same. As discussed in connection with the embodiments of
FIGS. 2A and 2B, the grooves/cuts of FIG. 2C are made a distance apart
first from leg portion 18, then from cut/groove 30. Preferably, leg
portion 18 and cuts/grooves 30 and 34 are substantially parallel one to
another, for reasons relating to available trimming technology and
calculation/monitoring of the increase of resistance value imparted by
each successive trim.
FIGS. 3A-3C represent various other embodiments of the present invention
and show incorporated therewith one (FIG. 3A) or a plurality of (FIGS. 3B
and 3C) of plunge cuts in conjunction with an initial U-cut and the
incremental cuts of this invention. As mentioned above, a plurality of
such plunge cuts 38 comprise a comb cut or an isolation comb cut, as would
be recognized by those skilled in the art. Such cuts can be utilized to
minimize arcing and are typically effective to about 300 volts per cut;
e.g., 10 cuts can be used if resistance to 3000 volts of lightning surge
is desired. Another representative embodiment of the present invention is
as shown in FIG. 4. Additional trimming to increase resistance value can
be accomplished by one or more J-cuts or J-grooves as shown therein.
Curved groove 40 can be initially made as shown in FIGS. 1-5 of
incorporated U.S. Pat. No. 5,043,694. As with other embodiments of the
present invention, the component illustrated in FIG. 4 can be prepared as
otherwise described herein. Grooves/cuts 30a and 30b, in combination with
the respective curved portions 32a and 32b do not extend into the current
pathway otherwise imposed upon resistor 24 by curved cut 40. One or more
plunge cuts of the type described herein can also be used in conjunction
with the resistor illustrated in FIG. 4, and used with the method by which
it can be prepared.
Another embodiment of the invention is illustrated in FIG. 5. In this
embodiment, an extended J-cut 42 can be used to trim additional material
from the resistive portion 24. The extended J-cut 42 is preferably used to
trim the value of the resistor component by as much as 3% of the total
value of the predetermined final resistance, while other embodiments of
J-cuts as described above are generally used to trim the value of the
resistor component up to about 1% of the predetermined final resistance
value.
The extended J-cut 42 can comprise an initial cut/groove 44, a curved
corner 46, and a curved connector 50. The cut/groove 44 extends into the
resistive portion 24 beyond the linear section 20 of the U-cut. The curved
connector 50 couples the extended J-cut 42 to the U-cut. Preferably, the
extended J-cut 42 further comprises a generally linear section 48 which
joins the curved corner 46 to the curved connector 50. In some
applications, however, the curved corner 46 and curved connector 50 can
form a single curved portion. The length of the initial cut/groove 44 and
the length of the generally linear section 48 are both variable to provide
various resistance trim levels. As in other embodiments of the invention,
one or more plunge cuts 38, extending from the edge of the trimmed portion
to the curved portion of the U-cut, may be cut into the trimmed portion
22. Furthermore, as is shown in FIG. 6, the resistor 16 may also be
trimmed with both an extended J-cut 42 and a cut/groove 30. The extended
J-cuts of this invention can also be used with a variety of initial cuts,
including but not limited to that shown in FIG. 4.
In preferred embodiments, the straight cut/groove 44 is preferably
substantially parallel to the leg 18 of the U-cut, while the generally
linear section 48 is substantially parallel to the linear cut 20 of the
U-cut. In these embodiments, therefore, the extended J-cut 42 generally
follows the contour of the U-cut. To minimize current crowding, however,
the extended J-cut 42 preferably extends only a relatively short distance
beyond the linear section 20 of the U-cut into the current pathway. In
highly preferred embodiments, the length of the cut/groove 44 is
substantially restricted to about two milli-inches beyond the linear
section 20 of the U-cut. Also to minimize current crowding, the curved
corner 46 and curved connector 50 preferably have a radius or degree of
curvature which is substantially equal to that of the curved connector 28.
Any of the available laser and monitoring technologies can be used to
practice the method and/or apparatus of this invention. To that effect,
U.S. Pat. No. 4,929,923 is incorporated by reference herein in its
entirety. The technology available under the acronym YAG and recognized by
those skilled in the art is especially suitable. In particular, the
commercially-available Teradyne W-series and CLS-37 YAG laser systems can
be used with results consistent with the scope of this invention. Such a
system includes the appropriate software and programming necessary to
control various trim parameters. For example, a programming algorithm is
incorporated to execute a curved cut. It is also desirable that the
velocity of the trim/cut must be held constant because varying laser trim
speeds can create heat stress within the resistor. The path of the
trim/cut must also be wide enough to minimize voltage stress.
For these reasons, other such programming, operational features are
preferably present and used with a component which continuously monitors
resistance value during the trim. When used with the present invention,
such a laser system can monitor resistance value during execution of the
leg portion/groove of a J-cut. Upon reaching a resistance within about
0.5% of the desired value, the system can begin executing a curved cut
having a radial dimension pre-determined in accordance with a curved
portion of an initial U-cut and with consideration of existing current
pathways.
As is evident from review of this summary and the figures thereof, the
present invention includes a resistor component and/or method of trimming
which provides for endpoints out of the current pathway. It is well-known
that the last laser pulse forming an endpoint can cause a region of
micro-cracking. Current flow can thereby cause weak spots in the resistor
and a drift of resistance value. The present invention alleviates this
problem by locating the endpoints of any cut/groove outside the resistive
portion or on a previous cut/groove, such that they are necessarily out of
the current pathway. Configuration of the grooves/cuts within a resistive
portion also incrementally increases resistance value.
EXAMPLES OF THE INVENTION
The following non-limiting examples show various aspects demonstrating the
utility of this invention. While such aspects, benefits and advantages are
shown with respect to certain embodiments, the same can be obtained via
other embodiments of this invention. In particular and in comparison with
the prior art, the precise, tight resistance tolerances available are
surprising and unexpected.
The data of Examples 1-3 was accumulated using a Teradyne W411 and a CLS-37
YAG laser system and the following parameters: Q rate, 4 KHz; Bite, 0.2
mils, speed, 0.8 in/sec, and laser power of 1.25 watts average power in
pulsed mode.
Example 1
A resistor component of the type illustrated in FIG. 3C was tested
experimentally under conditions simulating a lightning surge and
withstood, without breakdown or resistance drift, a voltage extreme of 2.5
kV.
Example 2
Reduced trim time is available through use of the present invention. For
purposes of comparison and using resistors trimmed to a value of about 200
kilo.multidot.ohms, a resistance trim of the prior art (U-cut plus a
piggy-back scan along the non-leg portion of the U-cut) was done in 172
seconds, while a trim according to this invention (U-cut plus a vernier
trim using a J-cut; reference is made to FIG. 2A) was accomplished in 79
seconds: a differential which represents a considerable savings of time
and production costs. It would also be observed that the piggy-back scan
cut of the prior art, in addition to necessitating additional time, leaves
endpoints proximate to existing current pathways.
Example 3
Traditional U-cuts overshoot or undershoot desired resistor values and
tolerances because of substrate and/or thick film ink imperfections,
either of which can cause non-homogeneous sheet resistivities. As a
result, single U-cut post-trim tolerances are typically .+-.0.5% of the
desired value. In contrast, and by way of further demonstrating the
utility of the present invention, the traditional U-cut can be used with a
J-cut/groove to provide post-trim tolerances of .+-.0.05%. Such precision
is provided without degrading surge power capability and without
increasing current crowding.
While the principles of this invention have been described in connection
with specific embodiments, it should be understood clearly that these
descriptions, along with any data provided, are made only by way of
example and are not intended to limit the scope of this invention, in any
manner. Other advantages and features of the invention will become
apparent from the following claims, with the scope thereof determined by
the reasonable equivalents, as understood by those skilled in the art.
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